Echinoderm skeletons

Echinoderm skeletons are constructed from a unique, intricately shaped, 3D, single crystalline meshwork with a topological structure in which every internal pore and channel is in direct contact with all others (periodic minimal surface). This property is likely to facilitate mass transfer and tissue developmeut (Aizenberg and Hendler, 2004). 

Sea urchin larvae are about a hundred micrometers in diameter. They have an internal skeleton that supports the soft tissues. The skeleton is composed of one or several pairs of intricately shaped spicules, the morphologies of which vary among different species. The spicule is composed of two different minerals, amorphous calcium carbonate and calcite [66]. The mineral phases in the adult skeleton are thought to be similar to those in the larval skeleton [67]. In fact almost the whole echinoderm phylum appears to use this type of material for constructing a large variety of skeletal elements. 

Another unexpected role of some skeletal pieces has been described. Ophiuroids are a large group of Echinoderms that includes the brittlestars. They have five arms, superficially resembling true starfishes (Asteroidea), and can catch fast motile prey. Aizenberg and coworkers have found that calcite crystals in their skeletons act as optical receptors or lenses. However, it is not known whether the system of lenses and nerves is actually an eye . However, these structures are absent in closely related but not light sensitive species of brittlestars. 

The echinoderms (phylum Echinodermata) include starfish, sea-urchins, sea cucumbers, and crinoids. A great many of these organisms were fossilized because they have skeletons made of calcite plates. The greatest number of different genera of echinoderms lived during the Carboniferous (360-286 million years ago). The embryology of modern echinoderms suggests that they are... 

Single cells may deposit CaCOs extracellularly. This occurs in the formation of the test in protozoa (Pautard, 1970) and the spicules of calcareous sponges (Jones, 1970). Single cells also have the capacity to form portions or entire skeletons of echinoderm larvae in vitro (Okazaki, 1975) and cells which do not form distinct epithelia regenerate spines of echinoderms extracellularly (Heatfield and Travis, 1972). These results suggest the possibility that cells which are not in an epithelial layer may also deposit the meshwork of the echinoderm test and spines extracellularly. 

Mineralized tissues of skeletons and shells of molluscs, annelids, barnacles, and echinoderms may be affected by an enzymic dissolution (digestion) of the organic matrix which contains calcium carbonate crystallites (Carriker and Smith, 1969 Carriker et al., 1969). An ability to digest organic matter from within animal skeletons has been demonstrated for bacteria (DiSalvo,... 

Starfish (sea stars), sea cucumbers, brittle stars, sand dollars, and sea urchins are members of a group of spiny-skinned animals known as echinoderms. All echinoderms are radially symmetrical, and most have five or more arms extending from a central disk, as shown in Figure 4.5. Their bodies are supported by internal skeletons which are solid in some species and jointed in others. Most echinoderms are designed so that their mouths are located on the ventral sides and their anuses on the dorsal surface. Depending on the species, the nutritional strategies of echinoderms vary from carnivores to detritivores or herbivores. 

Sand dollars are so flat that at first look they do not resemble their echinoderm relatives. The plates of their skeletons are fused and fixed, and the external surfaces are covered with tiny spines that look like fuzz or a short coat of hair. Sand dollars use their spines to burrow into sand. Once buried, the spines collect small food particles that fall among the spines and transfer them to the mouth. As shown in the upper color insert on page C-7, the common sand dollar (Echinarachnius parma) may be brown with purple or red tints. A five-petal pattern on the sand dollar s dorsal side corresponds to the five legs of a starfish. Holes near the tips of the petals allow the animal to extend tube feet, which are used for respiration. Most specimens measure about 3 inches (7.5 cm) in width. 

Biological systems provide numerous examples of micropatterned inorganic materials that directly develop into their intricate architectures, as illustrated by skeleton formation in echinoderms with component function of speciahzed photosensory organs [19,248]. Each skeletal structural unit (spines, test plates) is composed of a single caldte crystal delicately patterned on the micrometer scale, which is composed of a close-set array of hemispherical caldtic structures (40-50 pm in diameter) with a characteristic double-lens design (Fig. 20). 

In search of scaffolding materials, we have so far identified candidate biomatrices in nature, with varied chemical homologies and structural analogies to human extracellular matrices and whole tissues. They include nacre marine shell, marine sponge skeletons, echinoderm skeletal elements, and coral skeletons. The utility of selected species of these marine animals has been applied to the regeneration of human bone and cartilage. However, the full utility in these tissues and other tissues has yet to be harnessed and exploited. 

Hydrothermal or similar conversion of calcium carbonate to calcium phosphate processing of echinoderm structures as with other mineralized skeletons such as corals, transforms the chemical and mechanical properties with equivalence to human bone (Hu et al., 2001 Vago, 2008). 

The community of traditionally minded echinoderm palaeontologists, in contrast, has tended to take the echinoderm status of carpoids for granted, by virtue of the presence of a key character—the calcite skeleton—in these animals. Because of this, echinoderm palaeontologists have, in general, been unable to counter Jefferies thesis except with unsupported assertions that he is wrong, or detailed and untestable critiques of aspects of the anatomy of animals held to be fossil echinoderms (see Gee 1996, for a detailed commentary). 

I would prefer to take a middle view. Peterson s conclusion, though possibly correct, seems extreme, given the untested possibility that some of Jefferies interpretations, made over many years in a long series of papers, are more sound than others. Certainly, it is indefensible to insist that carpoids must be echinoderms simply because they have a calcite skeleton—a feature that need only be diagnostic of crown-group echinoderms. 

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